The Association of American Railroad (A.A.R.) recognizes various types of standard designed couplers. One of type, designated "F" by the A.A.R., has two interlocking lugs which prevent vertical movement between two engaged type "F" couplers. These lugs also prevent a pulled-out type "F" coupler which is engaged with another type "F" coupler from falling to the road bed. Engaged A.A.R. standard type "F" couplers are prevented from moving vertically relative to each other by the engagement of the guard nose in the interlocking wing pocket. The presence of a lower support shelf and an auxiliary interlocking lug that engages that shelf of an adjacent type "F" coupler further prevents vertical movement of the two engaged type "F" couplers relative to each other. However, the interlocking lugs on the type "F" coupler limit horizontal angling of mated "F" type couplers to a maximum of 3.degree. 45 minutes and further limit vertical angling to a maximum of 2.degree..
Another design of knuckle type coupler which is recognized by the A.A.R. is the type "E" coupler. The type "E" coupler does not have interlocking lugs similar to those provided on the type "F" coupler. Advantageously, mated type "E" couplers are capable of greater horizontal and vertical angling than the type "F" couplers, and under A.A.R. standards the "E" couplers may be horizontally angled at 13.degree. 30' and vertically angled at 4.degree. 43' . The basic "E" coupler does not have interlocking features similar to those provided for on the type "F" coupler and a pulled-out type "E" coupler in engagement with another type "E" coupler will fall to the road bed. Heretofore, it has been proposed to retain such couplers from separating vertically by the provision of one or more support shelves on both the "E" and "F" type couplers. The lower support shelf prevents a mating pulled-out coupler of either design from dropping to the road bed. The lower shelf design for type "F" couplers is now an A.A.R. standard in the United States, while the lower shelf design for type "E" couplers is also in common commercial use in interchange in the United States. Equipping an "E" type coupler head with an upper support shelf above the knuckle in addition to the lower support shelf below the knuckle offers maximum protection to the car and prevents vertical passing in either direction of a mated knuckle-type coupler. The double shelf coupler will further support or be supported by an adjacent knuckle-type coupler in instances where one of the couplers is pulled-out. Difficulty may also be encountered by interference between the leading edges of the upper shelves when angling takes place, particularly as the top shelves may approach each other sooner than desired due to coupler droop caused by wear in keys, wearplates, key slots and the like. At this coupler droop position and maximum vertical angling, the top surface of the leading edges of the upper shelf contact before contact is made between the front face of the couplers and knuckles, thereby imposing the initial heavy buff loads on the shelf.